Why not? You'd prefer to start him with something that kicks harder, that he's less likely to want to practice with, and that is more likely to cause him to flinch?

My son and I both use .243's. Neither of us is an "expert", yet somehow we've managed to take 4 deer with 4 shots. Bottom line is, if you hit them through the lungs with a .243, they'll be just as dead as if you hit them through the lungs with a .30-06. If you gut-shot a deer, it's gut shot whether you're using a .243 or a 7mm Rem. Mag., and the extra 4 hundredths of an inch of bullet diameter won't change that fact. Either way, you're in for a tracking job. The key with any round is to choose a good target and make an accurate shot. And, the key to making accurate shots is practice. Your son will practice a lot more with a soft kicking .243 than he will with a bigger round. Do that, and a .243 will work just fine.

My first rifle was a .30-06 when I was 15. I was small for my age, and that rifle kickedhard enough to cause me to flinch badly, at least until I grew into it a little more. Ididn't get around to going deer hunting until 25 years later, and my first few deer hunts were with a .30-06. I've taken 4 with 4 shots with it. But, when I got my son his .243, and shot it, I decided I wanted one too. And, unless I'm going after something bigger than a whitetail, that's what I'll continue to use. I like being able to watch the deer through the scope even after the shot. With the 06, the recoil always made me lose sight of the deer after the shot so I never knew how he reacted or which way he ran. With the .243, my scope never leaves the target.

Having gone 4 for 4 with both a .243 and a .30-06, I'd have to say they are both effective as deer cartridges. The .243 is definitively more comfortable to shoot. And, it will work effectively, even in the hands of "non-experts."